Introduction of Phosphorous to Metal-Organic Frameworks for Fire-Safe Polymers: From Synthesis to Application

Abstract

Metal-organic frameworks (MOFs) are a class of materials with highly ordered porous structures. They are frequently utilized as flame retardants (FRs) to enhance the flame retardancy of polymers due to their distinctive high specific surface area, excellent porosity, and thermal stability. However, MOFs are often employed as synergists in combination with other flame retardants that possess superior flame retardant properties, owing to their limited effectiveness when used in isolation. Phosphorus flame retardants and halogen flame retardants are common primary flame retardants with recognized efficient flame retardant properties. A significant advantage of phosphorus flame retardants is that they emit fewer toxic gases during combustion, and they are widely used as a key component in modern polymer flame retardants due to their properties. A prominent trend in recent years has been the use of phosphorus flame retardants as the primary flame retardant in combination with the use of MOFs as synergists for polymer flame retardancy. This combination is more effective in improving the flame retardancy of polymers during combustion by, for example, catalyzing the formation of a denser carbon layer, while at the same time enhancing the dispersion of the filler in the polymer, thereby reducing the impact of phosphorus-containing FRs on mechanical properties. The utilization of such flame retardants is an emerging trend, as evidenced by numerous studies that have demonstrated their effectiveness. This paper examines the application of various MOFs-P FRs in different polymer systems, analyzing their impact on flame retardancy and mechanisms. It also compares MOFs-P FRs with other commercial phosphorus-based FRs, discusses the current state of it, and explores future applications and challenges.

1. Introduction

Polymers have gained widespread use in daily life and engineering applications owing to their outstanding mechanical properties, good processability, and low cost. − However, owing to their chemical structure rich in such elements as carbon and hydrogen, polymers usually have poor thermal stability and high flammability. During combustion, the polymer generates significant heat and smoke and even releases toxic gases; not only does this increase the risk of fire, but it can also lead to injuries and environmental pollution. Industries such as the cable, automotive and construction industries demand high levels of flame retardancy from the polymers used and therefore need to upgrade the flame retardancy of the polymers for further use. Halogenated flame retardants (FRs) were one of the important FRs in the early days due to their low price and high flame retardancy, but they tend to generate carcinogenic substances like polybrominated dibenzofurans during combustion and are harmful to the environment and human health. The commonly used halogen-free FRs are phosphorus-based FRs, carbon-based FRs, and clay-based FRs. Phosphorus-based FRs are recognized for their excellence in flame retardancy and their use continues to be high every year. − Nevertheless, due to its large particle size and uneven morphology, it may affect its poor dispersion in polymers and poor compatibility. The resulting deterioration in the heat resistance of the material at elevated temperatures and the reduction in flame retardant properties can also lead to a reduction in mechanical properties, which further affects the flame retardant behaviour. Due to their excellent properties, metal-organic frameworks (MOFs) have been increasingly used in polymers in recent years as a new type of flame retardant, often in conjunction with traditional flame retardants, to address the problem of uneven mixing in polymers. ,

Since being discovered, MOFs have been used widely in modifying polymers. Among the most representative architectures are three prototypical frameworks: Zeolitic Imidazolate Frameworks (ZIFs) with tetrahedral metal clusters (e.g. Zn²⁺/Co²⁺) bridged by imidazolate linkers, Materials of Institute Lavoisier (MILs) featuring trimeric metal-oxo clusters (e.g. Cr³⁺, Fe³⁺) connected by carboxylate ligands, and Universitetet i Oslo (UiO) series characterized by zirconium-based [Zr₆O₄(OH)₄] secondary building units (SBUs) with 12-connected terephthalate linkers. These materials exhibit an extensive surface area coupled with uniform pore architecture, adjustable surface properties, and exceptional thermal resistance. These combined characteristics position them as promising candidates for diverse applications including molecular storage systems, catalytic processes, advanced separation technologies, and controlled release mechanisms in pharmaceutical engineering. , MOFs have been used in these areas for a longer period of time, and some have been commercialised, as they have been mass-produced in recent years. , In addition to their applications in the above fields, MOFs are becoming increasingly popular as new flame retardants due to their high thermal stability, rich variety of transition metals, and abundance of flame retardant elements and carbon sources, as well as the ease with which their structures and properties can be adjusted. Furthermore, MOFs can play active roles as flame retardants alone or act as sacrificed templates during the preparation of their derivatives to improve polymer flame retardancy while maintaining or improving mechanical properties. However, MOFs would have an influence on the uniformity and stability of the char layer due to their inherently poor structural stability, probably leading to rapid decomposition at high temperatures, which in turn is unable to effectively insulate oxygen and heat, diminishing the flame retardant effect. , Due to their high specific surface area and porosity, MOFs can, as in their applications in other fields, serve as multifunctional catalysts and work well with other types of flame retardants in polymer flame retardancy. It is possible to provide adequate phosphorus sources through the use of phosphorus flame retardants, which can generate a dense carbon layer with the aid of MOFs. , The incorporation of MOFs-P FRs into polymers for modification has been shown to produce composites with both excellent flame retardant properties and good mechanical properties. Figure illustrates the development of MOFs-P FRs for polymer flame retardancy in recent years.

1.

A few MOF-P FRs research accomplishments from 2017 to 2024. Adapted with permission from ref . Copyright 2017 Elsevier. Adapted with permission from ref . Copyright 2018 John Wiley and Sons. Adapted with permission from ref . Copyright 2019 American Chemical Society. Adapted with permission from ref . Copyright 2020 American Chemical Society. Adapted with permission from ref . Copyright 2020 Elsevier. Adapted with permission from ref . Copyright 2021 Elsevier. Adapted with permission from ref . Copyright 2021 RCS Publishing. Adapted with permission from ref . Copyright 2022 Royal Society of Chemistry. Adapted with permission from ref . Copyright 2023 Elsevier. Adapted with permission from ref . Copyright 2023 Elsevier. Adapted with permission from ref . Copyright 2024 Royal Society of Chemistry. Adapted with permission from ref . Copyright 2024 John Wiley and Sons.

Application of MOFs-P FRs in polymer flame retardancy is now thriving. It is obvious, as shown in Figure a, that the frequency of use of MOFs-P FRs in polymers has generally been increasing in recent years. And that the interrelationship of the keywords-identified hotspots in Figures b,c suggests that MOFs-P FRs are more commonly used in epoxy resins (EPs). Incorporating MOFs-P FRs can significantly influence the smoke suppression and mechanical characteristics of the resin, thereby affecting its flame retardancy, a theme worth considering. On the other hand, Figure d demonstrates a characteristic small-world network topology within the MOFs-P FRs research domain. Despite geographical dispersion across four institutions, the high edge density and reduced average path length suggest efficient knowledge-exchange pathways, which provide a structural foundation for the systematic development of MOFs-P FRs. Although there are many commercial applications of MOFs in other fields because of their excellent properties, and there is also a large-scale research on combining MOFs with phosphorus flame retardants in the flame retardant field, MOFs have not been applied on a large scale in polymers because they have been used in polymer flame retardancy for only seven years. Therefore, it is necessary to write an article focusing on the application of MOFs-P FRs, a potential commercial flame retardant in polymers, in order to promote further in-depth research. In this paper, we first introduce the various synthesis mechanisms of common MOFs-P FRs. Next, we examine how different MOFs-P FRs influence key fire safety metrics. These include UL-94 vertical burn test ratings and limiting oxygen index (LOI) values. We then analyze their effects on parameters derived from cone calorimeter testing, such as peak heat release rate (pHRR), heat release rate (HRR), total heat release (THR), toxic smoke release (TSR), total smoke production (TSP), peak smoke release rate (pSPR), yield of CO and CO₂ (YCO, YCO₂) and CO and CO₂ peak concentrations (PCO, PCO₂). Additionally, we evaluate the impact of MOFs-P FRs on mechanical properties and provide reasoned analysis for the observed changes in these parameters. Finally, the paper summarizes recent progress in commercializing MOFs-P FRs for polymer applications and highlights critical challenges that must be addressed to advance their practical adoption.

2.

(a) Dynamics of the number of articles produced, (b, c) network visualization and density visualization of keyword occurrences, and (d) visualization of co-authorship networks for MOF-FRs (2017–2024).

2. Synthesis of MOFs-P FRs

2.1. Direct Blending

The direct compounding method achieves the physical integration of phosphorus-based FRs within MOFs through thermal processing techniques such as melt blending, bypassing the need for chemical derivatization or complex synthesis protocols. By leveraging MOFs’ inherently high specific surface area, this strategy enables uniform dispersion of flame retardant agents across the porous matrix, thereby synergistically enhancing polymeric fire resistance through optimized component compatibility. While the method demonstrates industrial viability via compatibility with standard processing equipment and energy-efficient solvent-free operation, it concurrently faces challenges including phase segregation-induced discontinuous dispersion, insufficient interfacial adhesion that compromises mechanical integrity, and reduced flame suppression efficiency due to uneven active-agent distribution. This approach was used by Chen et al., Jin et al., Xu et al., Bi et al., and Ma et al. ,−

2.2. Nanocomposite

Nanocomposite technology is one kind of technique that combines nanoscale materials with matrix materials together to form new composite materials with new properties. With the addition of nano-enhancers, the mechanical property, heat stability, electric conduction performance, etc., of matrix materials will be greatly improved. Common nanocomposite methods include monomer intercalation, solution intercalation, melt intercalation, and other means. Compared with organic particles, inorganic particles usually contain fewer atoms and simpler structures; thus, in flame retardant, nanocomposite is often applied to inorganic flame retardants.

Phosphorus monomer possesses an abundance of flame retardant elements, which enables it to rapidly generate gas-phase reactions at high temperatures to reduce the incidence of fire. However, due to the size of the irregular shape, its poor dispersion in a polymer matrix seriously hinders its extensive use in flame retardant polymer materials. , Because of the high specific surface area of MOFs, it is full contact with phosphorus monomers. Meanwhile, its pore structure can also provide space for phosphorus-containing monomers and distribute uniformly, which can form a homogeneous nanocomposite structure using the monomer intercalation method. Its addition helps the modifier disperse uniformly in the matrix and improves the compatibility with the matrix; This process fundamentally involves the intercalation of organic MOF structures within the interlayer regions of inorganic phosphorus-based FRs, succeeded by in situ polymerization to establish a polymeric network. Such methodology ensures the homogeneous dispersion of nanoscale components throughout the polymer matrix. The synthesis mechanism of this type of flame retardant is shown in Figure a. Compared with the addition of phosphorus monomer only, the addition of MOFs can make full use of its properties as a multifunctional catalyst by virtue of its high specific surface area, forming a dense carbon layer in the substrate and generating PO• radicals in the gas phase. Combustion produces NH₃ and H₂O, which dilute flammable gases and enhance flame retardancy. In addition, better dispersion of modified particles improves the mechanical properties of composites. However, conventional nanocomposite approaches only partially exploit MOFs’ high surface area, while the irregular morphology of phosphorus monomers results in poor dispersion within polymer matrices, hindering their widespread application in flame-retardant polymeric materials. Yan et al., Xu et al., Liu et al., Song et al., and Bi et al. employed a nanocomposite method to synthesise MOFs-P FRs. ,,,,

3.

Synthesis mechanism of P-MOF FRs: (a) Nanocomposite. (b) Grafting. (c) In situ encapsulation. (d) Ligand substitution. (e) Adsorption. (f) Acid and alkali etching.

2.3. Grafting

The MOFs with hollow structures have higher specific surface area and porosity, which provides more exposed active sites and higher mass transfer efficiency, effectively trapping toxic gases and smoke particles to boost the fire resistance of polymers. However, in the synthesis or application process, the active sites on these hollow MOFs may be lost due to changes in external conditions, thus reducing their flame retardant properties it brings.

Grafting molecules onto MOFs is a widely used approach to enhance their flame retardant capabilities. Grafting functional groups containing flame retardant elements can enhance the material’s resistance to thermal degradation; simultaneously, molecules containing active functional groups can be immobilized on the surface or inside of MOFs, which can be used as new active sites to participate in the flame retardant process, enhancing the flame retardant effect of MOFs. The disadvantage of this method is that the graft modification process can be complex and requires precise control of the reaction conditions to ensure efficient grafting. Common grafting methods include the Kabachnik-Fields method, Click Chemistry, Schiff base reaction, etc. DOPO is an important organophosphorus compound used as a halogen-free flame retardant intermediate. It is rich in flame retardant elements; it can be used as a source of phosphonate diesters, and the imines generated by the reaction of carbonyl compounds with amine functional groups in MOFs can be used to generate new substituents for grafting onto MOFs by the Kabachnik-Fields method. This process is predominantly facilitated through the reaction of phosphorus-based FRs with MOF architectures under nitrogen inertion conditions. The grafting process is achieved via hydrophosphorylation addition between reactive moieties-specifically, the PH bonds inherent in flame retardant constituents and the CN double bonds characteristically embedded within MOF frameworks. The mechanism for synthesizing this flame retardant is shown in Figure b. Wang et al., Wan et al., and Li et al. applied this method in their studies. ,,

2.4. In Situ Encapsulation

Phosphorus-containing FRs frequently require synergistic additives due to their propensity for thermal degradation at elevated temperatures, which diminishes their capacity to suppress polymer combustibility. In situ encapsulation technology has become an effective solution, which is a means of confining a guest molecule in a pore or cavity in a host material. It can achieve effective encapsulation and controlled release of phosphorus flame retardants by utilizing the regular porous structure of MOF, which is an effective means of preventing their decomposition at high temperatures and accurately controlling the release rate. This approach not only protects the flame retardant from early decomposition but also can release sufficient flame retardant in time to offer continuous flame protection in case of fire, but the in situ encapsulation process can be complex, and the need to precisely control the porous structure of MOFs is a problem when using this method. It is very common to encapsulate phosphorus-based FRs, such as phosphonitrile and phosphate esters, into MOFs, not only because of their excellent chemical stability but also due to the adjustable pore size. , The in situ encapsulation procedure primarily involves initially dissolving organic ligands and phosphorus-based FR molecules in a solvent, with subsequent stirring to ensure homogeneous dispersion. This is followed by the gradual addition of a metallic salt solution to the mixture through dropwise incorporation. The reaction mixture undergoes continuous agitation at a controlled temperature, facilitating the formation of the MOF structure while effectively entrapping the flame retardant molecules within the crystalline matrix. The synthesis mechanism of such a type of flame retardant is shown in Figure c. Guan et al., Wang et al., and Song et al. synthesized MOF-P FRs through in situ encapsulation techniques. ,,

2.5. Ligand Substitution

Although the metal nodes in MOFs can catalytically affect the formation of carbon layers, the density of the formed carbon layer is too low to efficiently prevent the transfer of heat and oxygen, whereas the inherent microporous structure of MOFs results in weak adsorption performance in that regard, which in a cascading way affects the general properties of MOFs as fire suppressant agents. The geometry of MOFs can be effectively tuned by substitution reactions on the ligands. During or after synthesis, the stability and responsiveness of MOFs can be modulated through the introduction of functional groups onto the ligands using organic synthesis methodologies. Some organophosphorus flame retardants can be used as ligands to self-assemble through coordination, forming a class of crystalline materials with a multi-dimensional mesh structure. These materials can serve as a bridge to connect the metal centre and determine the pore structure and properties of MOFs. This allows for the design and synthesis of MOFs with different pore sizes and environments, enhancing their adsorption properties. The synthesis process involves combining phosphorus-containing flame retardants with metallic salts in a solvent medium, followed by ultrasonic treatment to ensure thorough mixing. The resulting mixture undergoes a controlled thermal reaction, culminating in the formation of a MOF structure. This framework is characterized by the substitution of its organic ligands with those derived from the phosphorus-based FR compounds, the synthesis mechanism of this type of flame retardant is shown in Figure d. The synergistic interaction between these ligands and metal nodes promotes the formation of thermally insulating carbonaceous layers, effectively impeding heat and oxygen transfer pathways. The introduction of functional groups to ligands requires careful consideration, as such modifications may compromise the structural stability and porosity of MOFs. The work of Yang et al. deals with this approach.

2.6. Adsorption

To improve the physical barrier and gas-phase flame retardancy of MOFs and phosphorus flame retardants, and to enhance the synergistic effect of the two, incorporating the two by adsorption is also a common means of improving flame retardant performance. MOFs have a highly ordered pore structure and possess a high specific surface area, offering many active sites for adsorptive processes. Despite the high specific surface area of MOFs, the molecular dimensions and geometry of phosphorus-based FRs critically govern adsorption efficiency, a parameter demanding prioritised analysis. Triethyl phosphate (TEP), an illustrative example, demonstrates pore compatibility with MOFs, where its molecular framework coordinates with metallic nodes or organic ligands, achieving immobilization within cavities. The preparation of this flame-retardant material involves introducing MOF powder into a phosphorus-based FR solution of a specific concentration. Ultrasonic treatment is applied to enhance adsorption efficiency, followed by controlled stirring to ensure thorough infiltration of the flame retardant into the MOF pore network. This type of MOF-P FR was subsequently obtained by centrifugal separation and drying. The synthesis mechanism of this flame retardant system is detailed in Figure e. This property of adsorption was utilized in the study of Zhao et al. This means of adsorption of phosphorus flame retardants by MOFs can efficiently apply the property of regular porous structure of MOF, and thus has certain commercialization potential.

2.7. Acid and Alkali Etching

Acid–base etching of MOFs represents a viable strategy for augmenting active site density, although precise modulation of etching parameters is essential to prevent structural compromise. Over-etching may induce pore framework collapse, while insufficient etching fails to optimise active site exposure. Phytic acid (PA) is a strong organic acid with multiple phosphate groups and can release protons (H⁺). These protons penetrate to the microporous structure of MOF and can complex with metal ions, which can induce the metal ions in the MOF structure to detach from the framework for the etching of the MOF structure. The synthesis protocol for this flame retardant system involves three sequential stages: initially dispersing MOF particles in an aqueous medium, followed by incorporating phosphorus-based compounds such as PA. Subsequent mechanical agitation enables comprehensive interaction between the flame retardant agents and the MOF framework, achieving surface modification. The final PA-functionalized MOF flame retardant is harvested through centrifugal separation and thermal desiccation. Its synthesis mechanism is shown in Figure f. Although PA itself is not easily used as a standalone flame retardant, the abundant phosphate groups within it can serve as a phosphorus source to increase phosphorus content in MOFs, thereby exposing more metal nodes. This adsorption mechanism leverages the inherent periodic mesoporosity of MOF frameworks to immobilize phosphorus-based FRs, thereby demonstrating scalable manufacturing potential for industrial fireproofing applications. The work of Zhang et al., Song et al., Bi et al. and Quan et al. all accomplished the etching of MOF. ,,,

2.8. Other Methods

The derivative method is a way of synthesizing MOFs-P FRs using hydrothermal treatment and substitution of organic linkers with phosphorus-containing ions. Leveraging MOFs’ inherent porosity and ordered pore architecture, this process creates hierarchical mesoporous structures in the resulting flame retardants. Synthesis involves treating precursor MOFs with phosphorus sources under controlled hydrothermal conditions, enabling ion exchange and structural transformation into rod-like frameworks with high surface areas. The mesoporous design enhances compatibility and dispersion in polymer matrices, while hierarchical pores act as nucleation sites for char residue, trapping smoke particles and reducing emissions. However, the multi-step protocol demands precise control of precursor synthesis, hydrothermal parameters, and structural evolution, limiting scalability due to prolonged processing times and operational complexity. Zhang et al. utilized this method to complete their study.

The post-synthetic modification (PSM) approach synthesises MOF-P FRs by chemically grafting phosphorus-based FRs onto MOF surfaces. This method involves dispersing synthesised MOF particles in a solvent, introducing phosphorus-based FRs, and stirring the mixture under controlled conditions to facilitate grafting reactions. The resulting MOF-P FRs are subsequently isolated and purified. Leveraging the high specific surface area and intrinsic porosity of MOFs, this strategy enables uniform dispersion of flame retardants within the porous matrix, enhancing polymer flame retardancy through improved component compatibility. Challenges include complex procedures, stringent reaction conditions, and scalability limitations due to specialised equipment and precise control requirements. Xu et al. completed the preparation of MOF-P FRs using the PSM method.

3. Application of MOFs-P FRs in Polymer Composites

3.1. Epoxy

EPs are commonly used in coatings, electronics and aerospace applications due to their mechanical, electrical, chemical stability and adhesion properties. , These applications can be risky due to the flammability of EP. Although EP is relatively stable at high temperatures due to its high ether bonding and high cross-linking density, the inherent hydrocarbon structure of the resin material makes it highly flammable, and it is, therefore, necessary to improve its flame retardancy in order to meet the needs of its applications in more fields. The epoxy group in EP can react with a loaded reactive phosphorus-containing group in MOFs-P FRs to form a more stable three-dimensional cross-linked network structure. Therefore, MOFs-P FRs are an important flame retardant for modifying EP; Table lists their applications in EP flame retardancy in recent years.

1. Summary of the Ability of MOFs-P to Act as FRs for Epoxy.

	Adding amount (%)	UL-94	LOI (%)	pHRR reduction (%)	THR reduction (%)	TSP reduction (%)	Ref
ZIF-8@HCCP	4.0	-	27.3	47.1	13.7	21.8
ZIF-8@DMMP	20.0	V-1	23.6	45.5	20.8	23.8
ZNs-B/CP	2.0	-	28.4	43.1	11.9	17.9
UiO-66-NH2-TEP	4.0	-	-	33.1	10.7	17.6
P-UiO-66-NH2	1.0	V-1	22.3	30.8	-	37.7
W-Zr-MOF-DOPO	3.0	V-0	32.2	57.8	39.8	36.1
IL@NH2-MIL-101	3.0	-	29.8	51.2	14.4	13.1
IRMOF-3-P/HPCP	3.0/6.0	V-0	32.0	46.6	36.4	27.8
PA-UiO66-NH2	5.0	V-1	29.1	40.6	-	41.4
ZIF-8@PZN/APP	3.0/18.0	V-0	31.0	80.8	65.8	-
CPPHS	2.0	-	27.6	40.1	22.8	38.8
PA-ZIF67@ZIF8	5.0	V-0	29.3	42.2	33.0	27.2
PN-MOF	6.0	-	29.8	44.6	44.0	36.2

Hollow-structured MOFs exhibit substantial specific surface areas and porosity, facilitating enhanced active site availability for flame retardant organic compound immobilization. In their study, Wang et al. synthesised a hollow Zr-MOF-NH₂ framework through Na₂WO₄ etching under acidic conditions, subsequently grafting DOPO onto the structure to develop W-Zr-MOF-NH₂ flame retardant. The pHRR, THR, and TSP of EP were reduced by 57.8%, 39.8%, and 36.1%, respectively, with the addition of 3 wt %, the LOI value increased from 24.6% to 32.2% and passed the V-0 rating in UL-94. This phenomenon arises from the intricate architecture of hollow MOF frameworks, the labyrinthine diffusion pathways impede the linear propagation of organic volatiles while retarding the synchronised emission of thermal energy and pyrolytic byproducts. Phosphorus-containing radicals in DOPO trap H•, OH• and O• radicals in the gas phase, interrupting combustion. Polyphosphoric acid produced by DOPO pyrolysis carbonises EP and forms a dense carbon layer with MOF, improving flame retardancy. Figure a shows the associated mechanism.

4.

(a) Scheme of the flame retardancy mechanism for W-Zr-MOF-DOPO/EP. Reproduced from ref . Copyright 2021 Elsevier. (b) Dual-phase synergic flame retardant mechanism of ZIF-8 and DMMP. Reproduced from ref . Copyright 2024 Royal Society of Chemistry.

Guan et al. evacuated synthesised ZIF-8 to remove guest molecules, subsequently impregnating dimethyl methylphosphonate (DMMP) into the material’s pores, resulting in the encapsulation of DMMP by ZIF-8 and yielding ZIF-8@DMMP. When 20 wt % ZIF-8@DMMP was added, the pHRR and pSPR of the EP were reduced by 45.5% and 28.0% compared to those before modification. The LOI value increased by 23.6% and reached the UL-94 V-1 rating. In the gas phase, DMMP is slowly released from the ZIF-8 pores, and the resulting PO• and PO₂• radicals can capture the H• and OH• radicals produced by the thermal degradation of EP, thereby inhibiting the combustion chain reaction. In the condensed phase, ZIF-8 and DMMP synergy form Zn₂P₂O₇, which improves the density and thermal stability of the char layer and effectively hinders oxygen and heat transfer. The relevant mechanism is shown in Figure b. The wave-transparent property of ZIF-8 is also improved due to its high porosity and uniform dispersion of large pores. The synthesized fire retardants are both cost-effective and straightforward to manufacture, making them ideal for application as substrates and dielectrics in the production of electronic devices with market potential.

Yan et al. synthesised metal-organic framework-3 (IRMOF-3) by solvothermal method and combined it with diphenylphosphinyl to form IRMOF-3-P (I-P) FR. Then, it was mixed with hexaphenoxycyclotriphosphazene (HPCP) to form organic-inorganic FR enhancing EP compatibility. When 6 wt % HPCP was added with 3 wt % I-P, the HRR and THR of EP were reduced by 46.6% and 36.4%, respectively, compared with those before modification, and the LOI value was increased from 24.5% to 32.0%, which achieved the V-0 rating of UL-94. Flexural strength was also improved by the addition of I-P compared to the addition of HPCP only. The phosphorus-containing molecular fragments released by HPCP combine with the macromolecular fragments produced by EP decomposition to form a stable expanded carbon layer, while ZnO produced by the decomposition of IRMOF-3 has a good catalytic carbon formation effect. This effectively prevents heat and mass exchange and inhibits the combustion of EP composites.

Zhang et al. functionalized UiO-66-NH₂ with PA as the coordinating ligand, yielding flame retardant PA-UiO-66-NH₂. The pHRR, TSP, and PCO of EP decreased by 40.6%, 41.6%, and 65%, respectively, after the addition of 5 wt % PA-UiO-66-NH₂ compared with that before modification, the LOI value increased from 25.6% to 29.1% and passed the V-1 rating of UL-94. During combustion, PA-UiO-66-NH₂ was able to interact with the early pyrolysis products of the modified EP, thereby trapping H•, OH• and O• radicals in the gas phase to inhibit the combustion chain reaction. The EP obtained by this means also has good mechanical properties and is green and sustainable due to the incorporation of biomass PA.

Song et al. chose to use ZIF-67 and cobalt compounds released from ZIF-67 as dual templates and used them for loading HCCP to prepare a novel flame retardant, CPPHS, for enhancing the flame retardant properties of EP. The addition of 2 wt % CPPHS resulted in a 40.1%, 22.8%, 26.1%, and 11.1% decrease in pHRR, THR, TSP, and YCO₂, respectively, of the modified EP compared with that of the pre-modified EP, and the LOI value increased from 22.3% to 27.6%. The synergistic effect of the Co–P–N elements in the CPPHS facilitated the formation of a dense, highly extended continuous carbon layer. The SEM images and Raman spectra of the formed carbon layer are shown in Figures a–l. In addition, as a transition metal, cobalt acts as a catalyst in the carbonization process of the EP matrix. It interacts with phosphorus-containing fragments to form phosphorus–cobalt compounds. These compounds facilitate the carbonization of the composite during combustion, thereby enhancing the flame retardant properties of the EP material.

5.

Digital photos of the char layers of (a, b) EP, (e, f) EP/CPPHS, and (i, j) EP/CP. SEM images of the char layers from (c) EP, (g) EP/CPPHS, and (k) EP/CP. Raman spectra of the char layers of (d) EP, (h) EP/CPPHS, and (l) EP/CP. Reproduced from ref . Copyright 2023 Elsevier.

Yang et al. synthesized MOFs, cobalt-based, known as PN-MOFs with P and N elements, through bicarboxylic acid ligands having phosphate and triazole structure, added to cobalt source (Co­(NO₃)₂-6H₂O). Putting in 6 wt % of PN-MOF made reductions of pHRR, THR, TSP, YCO, and YCO₂ by 44.6%, 44%, 36.2%, 23.4%, and 47.1% in EP, respectively. These results are plotted in Figures a–d. LOI value also improved, from 24.3% to 29.8%. This is due to the fact that the phosphorus radicals generated by the pyrolysis of PN-MOF burst out highly reactive radicals OH• and H• in the combustion chain reaction, which have a bursting effect and can effectively cut off the combustion chain reaction, thus playing an effective role in gas-phase flame retardancy. Products from PN-MOF pyrolysis also had excellent roles in catalytic oxidation and adsorption for CO, which made smoke less toxic. The synthesis strategy of this modifier makes full use of the high porosity of MOF, which provides a new way to fabricate highly efficient and environmentally friendly flame-retardant materials with commercialization value.

6.

Incorporating the data from the cone calorimeter tests, the figures illustrate: (a) HRR, (b) THR, (c) SPR and (d) TSP curves of neat EP and EP/PN-MOF composites. Reproduced from ref . Copyright 2021 Elsevier.

3.2. Polyurethane

Polyurethane (PU) has many applications in the construction industry, footwear and apparel because of its good abrasion, oil and chemical resistance. , The relatively low cross-linking density of PU gives it good toughness but also makes it more prone to decomposition at high temperatures, and the incorporation of MOFs-P FRs improves the flame retardancy of the substrate by forming a stable carbon layer that reduces the appearance of PU melt drops.

α-Zirconium phosphate (α-ZrP), as a significant type of stratified compound, is commonly used in polymer flame retardancy due to its excellent physical barrier and carbonation effect. Based on its negatively charged surface, Xu et al. prepared the novel hybrid material ZIF-8/α-ZrP for PU flame retardancy by using ZIF-8 to grow on the surface of α-ZrP and allowing Zn²⁺ to be anchored to the α-ZrP via electrostatic forces. The composite synthesis mechanism is shown in Figure a. The pHRR, THR, and TSP of the composites were reduced by 69.6%, 45.6%, and 40.5%, respectively, when only 2 wt % of ZIF-8/α-ZrP was added. In contrast to using solely α-ZrP, the ZnO that results from the breakdown of ZIF-8 significantly boosts the creation of a carbon layer atop PU. This layer acts as an enhanced physical barrier, effectively bolstering both the flame resistance and the smoke suppression capabilities of the PU composite.

7.

(a) Illustration of ZIF-8 modification of α-ZrP and the preparation of PUE composites. Reproduced from ref . Copyright 2018 John Wiley and Sons. (b) Schematic for the synthetic route of APP@SiO₂@UiO-66-NH₂. Reproduced from ref . Copyright 2023 Elsevier.

Wan et al. have initially fostered SiO₂ directly on ammonium polyphosphate (APP)’s surface and worked on grafting it with UiO-66-NH₂, leading to APP@SiO₂@UiO-66-NH₂ used for flame retardancy in thermoplastic polyurethane (TPU), schematic route shown Figure b. For this flame retardant, with 20 wt % of APP@SiO₂@UiO-66-NH₂ added, the THR, TSP, YCO, and YCO₂ diminished by about 87.8%, 87.7%, 52.4%, and 76.8% correspondingly, an improvement in LOI from 21.3% to nearly 29.9% observed. Facilitating this was the Zr element in UiO-66-NH₂ catalyzing the formation of a stabilized carbon layer in the TPU matrix during combustion, and phosphorus radicals made from APP degradation reacting with reactive radicals present, subsequently leading to termination in combustion chain reactions to cut down combustible gas production.

Chen et al. obtained copper metal-organic framework (MOF-Cu) as an APP synergist for TPU flame retardancy by mixing copper nitrate trihydrate and 1,3,5-benzenetricarboxylic acid. 0.0625 wt % MOF-Cu with 7.9375 wt % of APP resulted in the reduction of composite’s pHRR, THR, and TSP by 76%, 69.3%, and 77.8%. The LOI value increased from 21% to 27%, and the UL-94 reached the V-0 rating. The addition of MOF-Cu and APP modifies the thermal decomposition pathway of the matrix, which allows the material to decompose at lower temperatures, producing copper-based substances, phosphoric acid and amino compounds. These by-products contribute to the dehydration of the TPU, forming a carbon-rich residue and emitting CO₂, which enhances the overall thermal stability of the composite.

Liu et al. synthesised biomass dopamine (PDA), ZIF-67 and APP compounding was carried out into a novel ternary inorganic–organic hybrid flame retardant (A@P-Z) for enhancing the flame retardancy of rigid polyurethane foam (RPUF). The addition of an appropriate amount of A@P-Z resulted in the reduction of pHRR, THR, pSPR, TSP, and PCO of the composites by 38.6%, 45.9%, 19.2%, 53.4%, and 38.1%, respectively, and the LOI was increased from 19.7% to 25.6% and achieved a UL-94 rating of V-0, and the photographs during the UL-94 test period are shown in Figures a–d. The addition of A@P-Z to RPUF helps to form a strong, well-structured carbon layer, which is denser and more graphitised. The addition of PDA may help to form a more complex organic framework, which further enhances the strength of the carbon layer and achieves a better flame retardant effect. This approach effectively mitigates the inherent flammability of PU associated with its porous architecture, thereby substantially improving fire safety performance through the restriction of oxygen diffusion pathways and combustible volatiles. In addition, the harmonious relationship between A@P-Z and the RPUF matrix enhances the mechanical integrity of the composite, suggesting strong compatibility and effective interfacial bonding.

8.

Digital photos of: (a) RPUF, (b) RPUF/APP, (c) RPUF/A-Z and (d) RPUF/A@P-Z during UL-94 test. Adapted with permission from ref . Copyright 2023 Elsevier.

3.3. Polyurea

Polyurea (PUA) holds a big place in anticorrosive paints, coatings, seals, and in many such engineering sections owing to its first-rate chemical, abrasion, and impact defences. , But still, PUA itself catches fire with ease, ignites easily and burns continuously, and in the midst of burning, it makes hydrogen cyanide, CO with many other poisonous gases posing threats to human health hence, its flame retardant treatment is needed for hiking its flame retardancy. ,

Based on the remarkable effect of MOFs-P FRs on PUA flame retardancy, Song et al. focused on the use of ZIF-67 as a co-component of phosphorus flame retardants for PUA flame retardancy, firstly They synthesised two cobalt-based isomorphs, cobalt–cobalt layered double hydroxide (CoCo-LDH) and cobalt-based carbonate (CBC), using ZIF-67 as a self-template. Triphenyl phosphate (TPP) was subsequently immobilised on these matrices to yield the m-CBC-P@LDH flame retardant, with the synthetic protocol schematically presented in Figure a. The incorporation of 5 wt % of m-CBC-P@LDH reduced the pHRR and THR of PUA by 41.6% and 20.6%, respectively, and increased the LOI value from 19.1% to 22.6% compared with that before modification. Within the composite, cobalt ions facilitated the formation of stable cobalt oxides by promoting the cyclization and cross-linking of molecular fragments, thereby strengthening the integrity of the carbon layer. At the same time, the phosphorus element from the incorporated TPP played a role in the enhanced carbonization of the cross-linked organic structure. This additional carbonization contributed to the expansion and strength of the protective carbon layer during the thermal degradation process. The filler also showed better interfacial compatibility and dispersion with the polyurea matrix, which also led to a significant increase in the tensile strength and elongation at the break of the composites. Subsequently, following eco-adaptive processing of ZIF-67, they sequentially integrated arginine (Arg) and PA into the matrix, ultimately synthesizing the ZIF@Arg-Co-PA flame retardant through coordinated binding interactions. The addition of 5 wt % ZIF@Arg-Co-PA resulted in a reduction of pHRR, THR, and TSP, of the composites, were reduced by 43.8%, 32.3%, and 34.3% compared with that before modification, while the LOI value was further increased to 23.2%. The enhancement of the strength of the char layer by the polyphosphoric and phosphite acids generated from the decomposition of PA, which promoted the dehydration and charring of the PUA molecular chains, together with the cobalt compounds with hierarchical pore structure derived from ZIF-67, were the important reasons for the changes in the above data. The flame retardant mechanism of the composites is shown schematically in Figure b. Due to the biomass properties of PA and Arg etched with ZIF-67, the prepared composite material has better recyclability and meets the requirement of green materials in the new era. Enhanced interfacial coordination between the filler and the PUA matrix led to improved mechanical strength in the composites.

9.

(a) Schematic illustration for the preparation of m-CBC@LDH, m-CBC-P@LDH, CBC and CBC-P. Reproduced from ref . Copyright 2023 Elsevier. (b) Schematic of the Flame Retardant Mechanism for the PUA/ZIF@Arg-Co-PA composite. Reproduced from ref . Copyright 2024 American Chemical Society.

Bi et al. developed other alternative strategies to integrate ZIF-67 with phosphorus-based FRs. Their approach involved converting red phosphorus (RP) into two-dimensional hierarchical architectures, which were combined with ZIF-67 through nanocomposite fabrication, yielding a 2D RP@ZIF flame-retardant system. This was followed by the uniform embedding of ZIF-67 nanoparticles by in situ growth, the addition of 5 wt % of 2D RP@ZIF resulted in the reduction of pHRR, THR, and TSP of PU by 48.0%, 54.2%, and 22.2%, respectively, and the LOI was enhanced from 17.7% to 22.9%. They concluded that the polyphosphoric acid and cobalt oxides generated in the condensed phase during the combustion of RP and ZIF-67 promoted the carbonization of the PUA matrix and formed a dense carbon layer, thus providing a better physical barrier effect. The 2D RP@ZIF-generated PO• radicals can interrupt the radical chain reactions during combustion, inactivate the H• and HO• radicals, and the generated NH₃ and H₂O helped dilute the oxygen by diluting the combustible gases to inhibit the intensification of the combustion, and the related flame retardant and smoke inhibition mechanism diagram is shown in Figure a. They then added ZIF-67 as a template and PA as a dopant, and then aniline with (NH₄)₂S₂O₈ to synthesise a novel Janus nanosphere superstructure called PANI@Co-PA for PUA flame retardancy by polymerization-etching. The schematic diagram of the synthesis of PANI@Co-PA is shown in Figure b. The addition of PANI@Co-PA obtained by stirring the raw material for 2 h resulted in a decrease of 54.9%, 59.9%, 46.2%, and 53.1% in pHRR, pSPR, PCO, and PCO₂, respectively, and an increase in LOI value to 21.3% for the composites. The incorporation of PANI@Co-PA resulted in a decrease in the Co oxides produced by the transition metal Co element with PA react and thus form a solid carbon layer, and its good compatibility with the matrix produces a continuous barrier network, which effectively improves the quality of the carbon layer and better performs the flame retardant effect. The tensile properties are enhanced due to the strong interfacial bonding between PANI@Co-PA and the matrix. This strategy concurrently enhances the electromagnetic shielding efficacy of PUA, capitalizing on its intrinsic insulative properties to augment performance in such applications.

10.

(a) Schematic diagram of 2D RP@ZIF on PUA flame retardant smoke suppression. Reproduced from ref . Copyright 2024 Elsevier. (b) Schematic of the synthesis of PANI@Co-PA. Reproduced from ref . Copyright 2024 John Wiley and Sons.

3.4. Polylactic Acid

Polylactic acid (PLA) is a biologically derived, eco-friendly polymer derived from sustainable resources. It is recognized for its ability to decompose naturally and interact harmoniously with biological systems. Meanwhile, it has good plasticity and processability, showing great application values in packaging materials, textiles, agriculture, and other aspects. However, the ester group (−COO−) in PLA is prone to fracture under heating conditions, generating flammable gases and thermal stability is poor in this material. It should be upgraded for the sake of improvement in flame retardation properties.

Jin et al. prepared PLA/PAPP/ZIF-67 composites by melt blending PLA, piperazine pyrophosphate (PAPP) and ZIF-67 and compared the effect of ZIF-67 incorporation on PLA/PAPP versus pure PLA. The addition of PLA/4.8PAPP/0.2ZIF-67 significantly increased the LOI value of the modified matrix from the original 21% to 33%, and the UL-94 rating was successfully achieved to V-0, and the photographs taken during the burning process are shown in Figures a–c. The substantial enhancement in flame retardancy is further substantiated by the findings of the cone calorimeter test, wherein PLA/4.8PAPP/0.2ZIF-67 exhibited a decrease in pHRR of 31% and 18.1%, and a decline in THR of 8% and 8.4%, in comparison to pure PLA and PLA/5PAPP, respectively. The related data are plotted in Figures d–g, indicating that the addition of PAPP and ZIF-67 can effectively improve the flame retardant properties of PLA composites. The improved performance is due to the interaction of phosphoric acid and its polymeric form, which results from the thermal decomposition of PAPP, with PLA and ZIF-67. This reaction produces a strong expanded carbon layer. In addition, the highly porous framework of ZIF-67 and its ability to chemically absorb heat prolong the burn time of PLA, providing more time for evacuation and fire response.

11.

Burning process of (a) PLA, (b) PLA/5PAPP, and (c) PLA/4.9PAPP/0.1MOF in the vertical burning tests. (d) HRR, (e) THR, (f) mass and (g) CO₂ produce curves of PLA and its composites. Adapted with permission from ref . Copyright 2023 Elsevier.

Wang et al. prepared the composite flame retardant UiO@TEPx for PLA flame retardancy by encapsulating TEP in the cavity of UiO66-NH₂ after synthesizing UiO66-NH₂. Compared with pure PLA, the addition of UiO@TEPx decreased the polymer pHRR, THR, pSPR, TSP, and PCO by 14.2%, 9.8%, 50.5%, 29.5%, and 48.4%, respectively, and increased the LOI from 18.1% to 24.2% and achieved a UL-94 rating of V-0. During the heating process, the TEP molecules encapsulated in the UiO66-NH₂ cavities are continuously released and pyrolysed in the gas phase by “thermophilic release”, generating phosphorus-containing radicals to inhibit the combustion chain reaction, and the pore structure of the UiO@TEPx is maintained at elevated temperatures. This strategy effectively addresses the melt dripping phenomenon caused by the intrinsically low melt viscosity of PLA at elevated temperatures, achieving concurrent suppression of soot particulates and hazardous gaseous emissions during thermal degradation.

Zhang et al. used the synthesized Ni-MOF as a sacrificial template and subsequently converted Ni-MOF to hierarchical pore nickel phosphate (Ni-PO) by a facile hydrothermal method for synergistic APP as a flame retardant for PLA/wood fibre composites. The SEM images of the relevant flame retardants are shown in Figures a–d. The addition of 10 wt % of APP and 9 wt % of APP with 1 wt % of Ni-PO to the composites resulted in a reduction of pHRR by 35.8% vs 25.9%, THR by 25.8% vs 26.3%, and pSPR by 48.4% vs 55.6%, respectively. The LOI value of the composite material can be increased from 19.3% to 27.8% with the addition of either modifier, and both achieve a UL-94 rating of V-1. The researchers found that the incorporation of Ni-PO and APP into the composites was instrumental in the development of a carbon layer. The Ni-PO, with its multi-level porosity, offered ample sites for the nucleation and expansion of smoke particles, effectively trapping them within the char. Notably, Ni-PO helped offset some of the mechanical properties reductions that come with APP addition. The integration of 5 wt % Ni-PO and 5 wt % APP in PLA composites resulted in only a slight 2.5% decrease in tensile strength and a notable 14.2% increase in Young’s modulus compared to pure PLA, indicating a boost in the durability and lifespan of the composite materials.

12.

SEM morphology of (a) Ni-MOF and (b, c) Ni-PO and (d) TEM morphology of Ni-PO. Adapted with permission from ref . Copyright 2019 American Chemical Society. SEM images of (e, f) Fe-MOF, (g, h) P-Fe-MOF and (i, j) T-Fe-MOF. Adapted with permission from ref . Copyright 2022 Elsevier.

3.5. Polystyrene

Polystyrene (PS) is extensively utilized in the automotive sector, home appliances, construction and other fields despite its senior transparency, processability and chemical resistance. However, its benzene ring structure is prone to release combustible volatile substances during combustion, hence, it becomes essential to augment its flame resistance capabilities.

Xu et al. prepared the flame retardant PCT@MOF-NH₂ by PSM method by adding phosphoramidite trimer (PCT) after firstly synthesizing a cobalt-based MOF (MOF-NH₂). Over 40% reduction in pHRR value and above 31% reduction in THR value is seen and the YCO decreased more than 30% with 3.0 wt % addition of PCT@MOF-NH₂. Phosphorus and nitrogen can make fuel vapours and air absorb less in the gas phase. The phosphorus present in PCT reacts with oxygen and transition metals within MOF-NH₂, inducing catalytic carbonization that establishes a barrier layer to impede heat and mass transfer. Furthermore, this mechanism effectively suppresses chain reactions triggered by free radicals generated during the thermal degradation of PS combustion, thereby significantly reducing the emission of smoke and toxic gases. The tensile strength of PS composites was also significantly improved by the incorporation of PCT@MOF-NH₂.

Zhao et al. on the other hand, explored the effect of adsorption of TEP by different pore sizes of MOF regarding the fire resistance characteristics of PS. The transformation of microporous MIL-53 into macroporous MIL-53 was achieved by cutting the carboxylic acid-metal bond in MIL-53 by ammonia to produce controllable pores, and finally, T-Fe-MOF was prepared by adsorption by mixing with TEP, and the SEM maps of the related flame retardant are shown in Figures e–j. The addition of 3.0 wt % T-Fe-MOF reduced the pHRR, THR, PCO, and PCO₂ of PS by 24.5%, 12.7%, 40.6%, and 68.8%, respectively. The macroporous structure of MIL-53 was able to immobilise the TEP and avoid its migration and exudation. This allows the addition of a modifier to better catalyze the CO oxidation reaction, consume oxygen and release a large amount of CO₂, reducing the production of the toxic gas CO. Moreover, P-Fe-MOF at high temperatures promotes dehydration cross-linking and charring, forming a dense carbon layer, reducing oxygen supply and mass transfer, and achieving a delayed combustion process. This means of reaming can make full use of the regular porous structure of MOF and thus has some commercialization prospects.

3.6. Polypropylene

Polypropylene (PP) is a prevalent polymer type in both light and heavy industries. However, it lacks the capacity for self-retardation due to its absence of flame retardant elements. Unmodified PP typically generates substantial amounts of heat, which contributes to the sustenance of the combustion process and facilitates the propagation of fire. Therefore, flame retardant modification of PP is needed to expand its field of use to reduce the risk of fire.

Xu et al. synthesised Co-MOF via a solvent-based method and fabricated a PP/MPP/Co-MOF composite through melt blending hot-pressing with PP and melamine polyphosphate (MPP). The Co-MOF synthesis protocol is detailed in Figure a. 2 wt % Co-MOF and 28 wt % MPP were added to reduce the pHRR, pSPR, and TSP of PP by 43.1%, 57%, and 37.3%, respectively, and increased the LOI from 18% to 26.8% and achieved UL-94 rating of V-0. In Co-MOF systems, cobalt ions serve as catalytic centres that synergistically interact with polyphosphoric acid and melamine derivatives generated during the thermal decomposition of MPP. This collaborative mechanism facilitates the development of robust, compact carbonaceous structures during combustion. Such carbon layer consolidation effectively prevents the melt-dripping phenomenon caused by rapid polymer liquefaction in PP under elevated thermal conditions. In addition, the Co-MOF’s compatibility with the PP matrix contributes to an increase in both tensile strength and stiffness.

13.

(a) Preparation mechanism diagram for Co-MOF. Reproduced from ref . Copyright 2022 Elsevier. (b) ZIF-67/PP/EG/PA composite preparation process. Reproduced from ref . Copyright 2024 Springer Nature.

Quan et al. pioneered a solvent-free reactive extrusion strategy to synthesise ZIF-67 directly within a PP matrix. By integrating PA and expandable graphite (EG) into the system, they fabricated a multifunctional ZIF-67/PP/EG/PA composite material. The corresponding fabrication mechanism is detailed in Figure b. The pHRR, THR, and TSR of the composites were reduced by 21.2%, 6.4%, and 22.3%, respectively, when compared with PP/EG/PA. This reduction can be attributed to the fact that, during the combustion process, ZIF-67 contributes to the formation of a dense phosphorus–carbon network, which can serve as a protective layer to effectively inhibit the transfer of heat and combustible materials. This method of producing MOFs may be more appropriate for large-scale manufacturing due to its sustainability as well as higher production efficiency, which is conducive to commercialization.

3.7. Other Polymers

Li et al. formed MH@Fe-MOF-P FR for polybutylene succinate (PBS) flame retardancy by chemically grafting diphenylphosphine (DPP) onto Fe-MOF on the basis of Fe-MOF-functionalized magnesium hydroxide (MH) to obtain MH@Fe-MOF. The addition of appropriate amount, MH@Fe-MOF-P resulted in the reduction of pHRR, THR, and TSP of PBS by 82.1%, 53.2%, and 30.7%, respectively, relative to those before modification, and the LOI was enhanced from 21.0% to 29.6%, while the UL-94 rating of V-0 was achieved. Within the gaseous phase, the phosphorus-containing radicals generated by high-temperature pyrolysis of organophosphorus were able to capture the reactive radicals in the gas phase to reduce the generation of combustible gases. The MgO generated from the decomposition of MH in the condensed phase works together with the Fe-MOF-derived carbon material to form a dense carbon layer to improve the strength and thermal stability of the carbon layer, thus providing better thermal and oxygen insulation. Incorporating MH@Fe-MOF-P into PBS resulted in a 22.9% rise in tensile strength, a 208.4% increase in elongation at break, and an 87.0% enhancement in impact strength compared to the use of MH alone. -NH₂ on Fe-MOF may undergo an amine-ester exchange reaction with the PBS chain, and the hydrophobic benzene ring on the outer layer of MH@Fe-MOF-P may also hinder the self-aggregation of the filler, thereby enhancing the compatibility and interfacial adhesion between the polymer and filler.

To enhance cotton fabric flame retardancy, Bi et al. synthesised polyphosphazene (PZS) microspheres from hexachlorocyclotriphosphazene and tannic acid, then sequentially immersed cotton fabric in UiO-66 precursor solution and a starch-PZS suspension, fabricating a cotton/UiO-66/St-PZS composite. This reduced the pHRR and THR by 46.6% and 31.7%, respectively, and improved the LOI from 17.2% to 24.8% than that before modification. The Zr element in UiO-66 can react with tannic acid by forming ZrO₂ during combustion to form a stronger and more stable carbon layer, which, together with the ability of PZS to capture free radicals, enhances the flame retardancy level of the composite. The tensile strength of the composites was also increased by 15.1% compared to the pre-modification level, one of the reasons for this is that the modifier forms a homogeneous cover layer on the surface of the cotton fibers, which provides an additional stress transfer path during the tensile process, which enhances the inter-fibre interactions and thus improves the overall mechanical properties of the cotton fabrics.

Ma et al. employed MOF-P FRs for enhancing the flame resistance of polyethylene terephthalate (PET). Through solvothermal synthesis of MIL-88B (Fe), they incorporated both the MOF and 3-hydroxyphenyl-phosphinyl propanoic acid (CEPPA) via physical blending during PET polycondensation, producing MIL-P-PET composites. The 0.6MIL-P-PET formulation containing 0.6 phr MIL-88B and 4.5 phr CEPPA demonstrated significant improvements: the LOI increased from 22% to 30%, UL-94 rating elevated from no rate to V-0, with concurrent reductions of 17.9% in pHRR and 14.3% in PCO₂. Mechanistic analysis revealed that MIL-P-PET developed a dual-layered char structure under thermal degradation, featuring a coarse exterior and compact interior, which effectively enhanced flame retardancy. The coordinated ferric ions promoted decarboxylation of PET ester groups, generating non-combustible CO₂ and terephthalic acid derivatives. This dual mechanism facilitated gas-phase flame inhibition through non-flammable gas release and condensed-phase protection via stabilised char layer formation, collectively improving fire performance.

4. Commercialization Value of MOF-P FRs

In recent years, MOFs have been combined with various commercially available phosphorus-based FRs to form MOF-P FRs for use in various types of polymers, with many excellent results. As can be seen from Table , after some commercial phosphorus flame retardants have significantly improved the flame retardant properties of polymers, the combination of MOFs and these phosphorus flame retardants as MOF-P FRs can further improve the flame retardant properties of these composites. At the same time, the incorporation of MOFs can also compensate for the decrease in mechanical properties of polymers due to the addition of phosphorus flame retardants. In addition, the addition of MOFs is conducive to the production of green flame retardant polymer products. Together, these factors allow these polymers to have wider applications in many commercial sectors. However, as MOFs have only been used as flame retardants in polymers for 7 years, there is still a long way to go in the commercialization of related products in polymer flame retardancy. The first and foremost challenge is the relatively high cost of raw materials to synthesise MOFs and the complexity of the process, which are difficulties that MOF-P FRs will have to overcome on their way to commercialization. Fortunately, MOFs have seen significant development in recent years, with companies like BASF successfully bringing some MOF products to market. In other fields, composites containing MOF have been prepared by researchers to synthesise scales of several kilograms for mass production. In the field of polymer flame retardancy, the preparation of MOFs directly from polymer matrices using solvent-free reactive extrusion has also been explored, which has been shown to have large yields in terms of time and space. , With the increasing attention given to MOF-P FRs in recent years, there is a good chance that such products could replicate the commercial success of graphene.

2. Summary of Flame Retardant Properties and Applications of Phosphorus-Based FRs and MOF-P FRs.

Polymer systems	Type of FRs	Loading (wt %)	Main flame-retardant results	Cost and scale	Ref
EP/DETA	APP	21	27.0% of LOI; UL-94 V-0 rating; 77.5%, 69.9%, 48.4%, 56.9%, and 84.0% reduction in pHRR, THR, pSPR, PCO, and PCO2 (heat flux of 50 kW/m2)	Low-cost and commonly used commercial flame retardant
	ZIF-8@PZN/APP	3/18	31.0% of LOI; UL-94 V-0 rating; 80.8%, 65.8%, 72.6%, 64.7%, and 85.3% reduction in pHRR, THR, pSPR, PCO, and PCO2 (heat flux of 50 kW/m2)	Relatively high-cost and bench-scale flame retardant
TPU	APP	20	28.8% of LOI; 69.8%, 69.5%, 60.0%, 23.8%, and 72.0% reduction in pHRR, THR, TSP, YCO, and YCO2 (heat flux of 35 kW/m2)	Low-cost and commonly used commercial flame retardant
	APP@SiO2@UiO-66-NH2	20	29.9% of LOI; 75.1%, 87.8%, 87.7%, 53.4%, and 76.8% reduction in pHRR, THR, TSP, YCO, and YCO2 (heat flux of 35 kW/m2)	Relatively high-cost and bench-scale flame retardant
PUA	RP	5	18.6% of LOI; 27.3%, 43.8%, −3.9%, 18.5%, and 36.7% reduction in pHRR, THR, pSPR, TSP, and PCO2 (heat flux of 50 kW/m2)	Low-cost and commonly used commercial flame retardant
	2D RP@ZIF	5	22.9% of LOI; 48.0%, 54.2%, 20.4%, 22.2%, and 49.0% reduction in pHRR, THR, pSPR, TSP, and PCO2 (heat flux of 50 kW/m2)	Medium-cost and bench-scale flame retardant
PP	MPP	30	25.5% of LOI; UL-94 V-0 rating; 45.0%, 22.6%, and −145.3% reduction in pHRR, pSPR, and THR (heat flux of 25 kW/m2)	Medium-cost and commonly used commercial flame retardant
	MPP/Co-MOF	28/2	26.8% of LOI; UL-94 V-0 rating; 43.1%, 57.2%, and 37.3% reduction in pHRR, pSPR, and THR (heat flux of 25 kW/m2)	Medium-cost and bench-scale flame retardant
PP	EG/PA	≥10	-	Medium-cost and commonly used commercial flame retardant
	ZIF-67/EG/PA	5/≥10	21.1%, 6.4%, 28.6%, and 22.3% reduction in pHRR, THR, pSPR, and TSP (heat flux of 50 kW/m2) compared to PP/EG/PA	Medium-cost and with relatively high temporal and spatial yields

^aLow-cost: The cost of FRs for each kilogram of composite material produced is about <$10. Medium-cost: The cost of FRs for each kilogram of composite material produced is about >$10 and <$50. Relatively high-cost: The cost of FRs for each kilogram of composite material produced is about >$50 and <$200.

Another major obstacle limiting the use of MOFs in polymer flame retardancy is that their regular porous structure is underutilized. This property gives the pore size and pore structure of MOFs the property that can be designed and regulated according to the needs, which provides the possibility of optimizing their performance in specific applications and has a lot of room for development. In recent years, some studies on MOFs in polymer flame retardancy have also made a lot of attempts to address this point. In this context, the combination of encapsulation and adsorption with phosphorus-based FRs can effectively utilize the porous structural properties of MOFs to provide a larger space for phosphorus-based FR loading, thus increasing the flame retardant content in the composite material. This not only helps to mitigate the effect of high temperature on phosphorus flame retardants, but the large pore structure also promotes the uniform distribution of flame retardants in MOFs and prevents the aggregation of phosphorus flame retardants in the polymer matrix. All these help to catalyze the formation of a denser carbon layer, which further improves the flame retardant performance. Etching to enlarge the pores is a new trend to take advantage of this property, the relevant means and specificities are shown in Figure . , Specifically, MOFs need to be expanded to improve their performance due to the limitations of their microporous structure in terms of adsorption and catalytic efficiency. The purpose of reaming is to improve the adsorption capacity of MOFs for large-size combustible gas molecules and smoke particles while avoiding the problem of transition metal-catalyzed polymer degradation. Pore enlargement can be achieved by post-treatment methods such as salt hydrolysis, ligand substitution, acid and alkali etching, etc. these methods can adjust the pore structure of MOFs, improve their adsorption capacity and catalytic efficiency, and also better combine with phosphorus flame retardants, which can help improve the flame retardant and mechanical properties of composites. Currently, there is an effective method for pore expansion is to construct hollow MOFs with open and complex nanostructures that have nano-openings in the walls for free access to release phosphorus-based FRs. However, this type of controlled preparation is more difficult to achieve due to the inherent properties of MOFs compared to other nanomorphologies. To overcome this problem, work has been carried out on loading phosphorus-based FRs while half-etching the MOFs by incorporating phosphoryl chloride and amino groups to react with the MOFs, which retains a high specific surface area and improves the adsorption capacity of large flammable and fumigant gases. The surface functional groups can improve the interfacial compatibility with the substrate and promote its dispersion in the substrate. This efficient use of the regular porous structure of MOFs gives this type of flame retardant commercial properties. , With an in-depth understanding of the structural properties of MOFs and the emergence of innovative application methods, the potential of MOF-P FRs in the field of polymer flame retardancy will be further explored and utilized, promoting the commercial development of this type of flame retardant.

14.

Schematic diagram of the pore control method and its advantages. Reproduced from ref . Copyright 2024 Elsevier.

While MOF-P FRs demonstrate promising flame retardancy and mechanical reinforcement, their environmental safety, particularly regarding heavy metal residues, requires critical evaluation compared to conventional phosphorus-based FRs. Most MOFs incorporate transition metals as coordination centres, which may pose leaching risks during polymer degradation, recycling, or improper disposal. Although some commercial phosphorus-based FRs have been criticised for bioaccumulation or toxic gas emission during combustion, the use of MOF-derived heavy metals could result in long-term soil and water contamination if not properly stabilised. Recent research advances have focused on the following strategies to address this problem: Developing MOFs with low-toxicity metals such as Al³⁺ or biomass-derived ligands to minimise environmental footprints. Aluminium-based MIL-96­(Al) and zirconium-based UiO-66 have demonstrated comparable flame retardant efficiency with reduced leaching risks. Furthermore, post-synthetic exchange with phosphorus-based ligands presents a viable strategy to address these challenges. A representative methodology involves substituting the 1,3,5-benzenetricarboxylic acid ligands in Ni-MOF with phosphate anions derived from Na₃PO₄, yielding hierarchically mesoporous Ni-PO. This approach enhances the flame-retardant efficacy of MOFs whilst effectively mitigating heavy metal leaching, which causes a reduction in Ni²⁺ migration under acidic conditions compared to pristine Ni-MOFs. Furthermore, there is a need for the prioritization of lifecycle assessments (LCA) and end-of-life management protocols for MOF-P FRs. While challenges persist, the integration of green chemistry principles and circular economy models will be pivotal in reconciling the flame retardant efficacy of MOF-P FRs with environmental sustainability.

5. Conclusion and Outlook

This paper reviews the combination of MOFs and phosphorus flame retardants, i.e., MOFs-P FRs, in polymer flame retardancy. Firstly, several common synthesis methods are introduced, then the flame retardant modification and flame retardant effects of MOFs-P FRs synthesis by different methods in different polymers are introduced, and finally the prospects of their commercial development are introduced, which shows that MOFs-P FRs play an important role in improving the flame retardancy of polymers and they have full potential for commercialization.

MOFs, due to their large specific surface area, adjustable pore structure, and excellent heat resistance, can significantly enhance the flame retardancy of various polymers when combined with phosphorus-based FRs, with minimal impact on the mechanical properties of the materials. MOFs-P FRs have good dispersibility and compatibility with polymers, and can form a persistent char layer during combustion, reducing heat and mass transfer. Their high specific surface area and porous structure can also adsorb smoke and toxic gases produced during burning, enhancing smoke suppression effects. In terms of mechanical properties, the addition of MOFs significantly compensates for the mechanical properties of the polymers when only phosphorus-based FRs are added, as the organic linkers of MOFs are well bonded to the polymer chains. MOF-P FRs are also characterized by the sustainability of the modified materials and the increasingly sophisticated preparation processes of recent years, all of which make MOF-P FRs an increasingly important next option in commercialised flame retardants.

Although MOFs-P FRs show significant performance advantages over traditional flame retardants, especially in terms of environmental protection and catalyzed carbon layer formation. However, there are still many challenges on the way to commercialization. One of the key points is that although the production of MOFs has been industrialised, the cost is still high compared with traditional flame retardants, and a systematic cost assessment system for the production of MOFs needs to be established, taking into account various process parameters including scale, raw materials as well as the cost of recycling and cleaning to promote further development. Although the porous structure of MOFs has been applied in a large number of studies on MOF-P FRs in recent years, there is still much room for the development of the unique characteristics of this structure, so it is a problem to be solved by further investigating the effect of the pore structure of MOFs on the migration and distribution of phosphorus flame retardants, and searching for a more suitable phosphorus flame retardant to correspond with it to make further use of this characteristic. Equally important is the fact that multifunctional flame retardants will be the future direction of development, and attention will be paid to whether the incorporation of MOFs-P FRs can be equally effective in improving the storage, separation and electromagnetic properties of polymer gases to produce composites that can be used in a wide range of applications. Finally, the heavy metal contamination risks associated with MOF-P FRs necessitate rigorous evaluation. The development of low-toxicity MOFs and strategies such as post-synthetic ligand exchange with phosphorus-containing ligands can mitigate heavy metal leaching risks. Prioritizing LCA and end-of-life management protocols is crucial for ensuring their environmental sustainability. Combining MOFs with phosphorus-based FRs offers an impactful strategy for improving the flame retardancy of polymers. As research and development efforts continue, MOFs-P FRs may take a solid step towards commercialization by striking a balance between performance in the field of flame retardancy, reasonable cost, sustainability and high utilization.

The authors declare no competing financial interest.